The present invention relates to a technique for producing holograms, including both the apparatus for doing so and the method of utilizing that apparatus.
Various techniques for producing holograms are known. These generally involve creating interference patterns or gratings in a surface which, when viewed, give rise to holographic visual effects. The conventional way to make these interference patterns is to selectively expose to illumination regions of a photosensitive material (photo-resist), which becomes insoluble in the exposed regions. The unexposed regions are then dissolved away, leaving behind the desired pattern. This pattern can then be regarded as the final holographic image. More commonly, however, it is used as a xe2x80x9cmasterxe2x80x9d to mechanically impress an equivalent pattern onto a separate surface, which then provides the desired holographic effect. Various auxiliary operations are also commonly performed. For example, the master can be metalized to strengthen its surface before using it to make the separate mechanical impression or impressions. The final holographic surface can also be metalized to provide increased light reflectivity to the viewer. Also, the pattern of illumination exposure can be reversed, provided the photosensitive material is one which becomes soluble through exposure, rather than the reverse.
More recently, an additional feature has been added to these conventional techniques. This feature involves treating the overall holographic image as an aggregate of, separate areas, called pixels. The illumination of the photosensitive material is then performed one pixel at a time until the whole image area has been covered in this manner. By illuminating each pixel with appropriately directed interfering light beams, the holographic properties of individual pixels are determined. In turn, this pixel-by-pixel treatment has led to the use of computer programs to control both of the interfering directionality of the illumination and the locations of the individual pixels. This enables the production of holograms which are more susceptible to design variations through adjustment of the computer programming so as to create pixel arrays with various holographic characteristics. Such a computerized technique is disclosed in U.S. Pat. No. 5,822,092. The resulting hologram, which consists of an array of holographic pixels, or dots, is sometimes referred to as a xe2x80x9cdot matrixxe2x80x9d hologram.
Although this prior art technique has some desirable features, it is far from ideal in several respects. First and foremost, it still relies on the process of polymerization of the photosensitive material in creating the interference pattern, or grating at each pixel location. This is inherently time consuming and must, of course, be followed by separate processes for removing the unpolymerized portions of the material. Such removal typically involves so called xe2x80x9cwet chemistryxe2x80x9d, which is another drawback. In addition, the technique requires the illumination to dwell on each pixel long enough for the polymerization process to become effective. That is not necessary if the illumination is performed, as was previously done, over large areas, or even over the whole image area at the same time.
The relatively time-consuming nature of this prior technique tended to discourage the creation of large-area holograms, and also the use of the initially created hologram as the final end product. Rather, it encouraged converting the initial creation into a master, for use in reproduction in the same manner as in prior non-pixel techniques.
Thus, any potential benefits of the computer-controlled pixel-by-pixel techniques are to a large extent negated. A major improvement over the technique described above is disclosed in prior U.S. patent application Ser. No. 09/021,281, filed Feb. 10, 1998, which is assigned to the assignee of the present invention and is incorporated herein by reference as if set forth at length. In essence, this improvement resides in discarding the use of photosensitive material, which must be illuminated to polymerize selected areas, in forming the desired interference patterns. Instead, a laser is used to ablate a workpiece so as to directly form each pixel of the desired overall holographic image. No exposure of photosensitive material is required and no post-exposure treatment is needed to produce the desired pattern.
However, even this improved technique can be still further improved. The above-identified prior application teaches the use of an interferometer head, which splits the laser beam into at least two parts, and then uses a set of angled mirrors to reunite these parts at the surface of the workpiece on which the desired interference patterns are formed through ablation. The azimuthal orientations, xcex1, at which the two beam parts reach the workpiece surface determine the direction of viewing at which the strongest holographic effect is perceived. The included angle, xcex2, between the beam parts reaching the workpiece determines the perceived holographic coloration. In order to produce different effects at different pixel locations, the aximuthal orientation of the whole interferometer head relative to the workplace surface has had to be changed intermittently and so did the angular orientation of the individual mirrors, which form part of this interferometer head. For example, if the viewing direction at which the strongest holographic effect from a given pixel is perceived (hereafter called the xe2x80x9cmaximum holographic directionxe2x80x9d) was to be changed by 90 degrees from one pixel to the next, then the whole interferometer also had to be reoriented in azimuth by 90 degrees. It is desired to make such changes very rapidly so as to enable the rapid formation of different holographic effects at consecutive pixel locations. This rapidity is especially crucial in the production of large area holograms by means of the pixel-by-pixel technique, since these require the formation of many individual pixels and therefore also potentially many changes in interferometer head orientation.
An interferometer head such as described above has substantial mass and inertia and is therefore difficult to reorient with the desire rapidity. Moreover, the mechanical movements, which are involved require precise location control and stability. As a practical matter, this limits the acceleration and deceleration rates for any head movement. Thus, the rate of creation of the different pixels which, in the aggregate, constitute the overall holographic imagery, becomes limited by the speed of reorientation movement of the interferometer head and its constituents. Thus, it is difficult to exploit the capability of the ablating laser in performing this pixel creation at the necessarily higher rates. Also, any vibration involved in these re-orientations can detract from the extremely high positional accuracy, which is desired in order to yield xe2x80x9cgoodxe2x80x9d holographic imagery.
The present invention uses a laser beam to address a workpiece at predetermined locations so as to form holographic interference patterns in the workpiece. To create the desired interference patterns the laser beam is split into a reference beam and at least one object beam. These beams are directed by an optical system so as to reunite or interfere at a pixel location on the surface of the workpiece. The beams arrive at each such pixel from controllable azimuthal directions and with controllable included angles. Thus, the interfering beams produce pixels with controllable maximum holographic direction and controllable coloration.
One of the optical components which is involved in providing this control of direction and coloration, and which does not remain stationary during the pixel creation, is at least one beam deflector, such as the electronically controllable mirrors of a multi-axis galvanometer. These beam deflectors are computer controlled to produce the desired holographic effect at each pixel location.
The beam deflectors used in accordance with this invention have an inertia, which is very low. Therefore, the holographic effects produced at different pixels can be changed much more rapidly than was possible with the prior art technique using high-inertia interferometer heads. Also, in the present invention, the danger of optical misalignment, which existed in the prior art, is greatly reduced.